Halacha states that the Tefillin must have four Parshiyot (Shema, Vehaya Im Shamoa, Kadesh, & Vehaya Ki Yeviacha), and if even one Parasha is missing, the Mitzvah is not fulfilled thereby. When Bnei Yisrael left Mitzrayim, they were told the two Parshiyot of Kadesh & Vehaya Ki Yeviacha, both of which command us to wear Tefillin. But the two Parshiyot of Shema & Vehaya Im Shamoa, were not given until 40 years later. So how did they fulfill this Mitzvah which requires these four Parshiyot when at the time they only had two of the four Parshiyot?

The simple answer is that the Torah itself states (Shmot 13, 5) that Tefilin are only to be laid after we enter the land. (See Chezkuni ibid 9) (The Talmud Bechorot 5a infers [Shut CHidushei Harim YD 19] that Tefilin were laid in the desert, however this may be only after all four Parshiot were given)